Modern trends in High Power Microwave (HPM) sources for a variety of applications are directed towards increasing radiated power and efficiency in order to increase energy density (energy per volume). Transmission line type pulse generators with photoconductive switches can achieve some of the best results. In particular, they are compact and provide fast pulse rise time, high power, and are suitable for generating short pulses (nanoseconds range) with perfect shape.
For a given charging voltage for transmission lines, limited by electric field in photoconductive switches, high powered and high energy density transmission lines imply low characteristic impedances. For bipolar pulse generators having transmission lines with low characteristic impedances (in the mOhm range), a problem occurs when the generators are used to drive typical load impedances, such as 50 Ohm or higher. This problem is generally addressed using impedance transformers to transform impedance from a low value in the bipolar pulse generator to a higher value presented by the load. It is reasonable to define the required impedance transformation as a ratio of specified load impedance and the lowest characteristic impedance of transmission line in pulse generator.
There are known efficient transmission line bipolar pulse generators, which provide high energy pulses without or with very limited impedance transformations. In addition, there are known impedance transformers of two categories suitable for transformation of short pulses:                Stepped (multi-sectional) and non-uniform transmission-line transformers (Tchebyscheff, exponential and others types)        Transmission-line transformers with series-parallel interconnected lines. In general, transformers of first category when used to transform from mOhm impedances to, for example 50 Ohms, generally have low efficiency, large size and are also difficult to fabricate. Therefore, these transformers themselves can defeat some of the advantages of having high efficiency, high powered and high energy density transmission line bipolar pulse generators.Transformers of the second category are more efficient, but have, in principle, shunt inductance(s) or short-circuited (on a far end) transmission line(s), i.e. inductive stub(s) that deteriorate the pulse shape.        
The final result is that known lower energy bipolar pulse generators with efficient impedance transformation, are more compact and provide about the same energy as higher energy bipolar pulse generators with moderate impedance transformation. See, for example, the co-pending patent application by Simon London, entitled BiPolar Pulse Generator With Voltage Multiplication,” filed on Nov. 9, 2005 and assigned application Ser. No. 11/269,847.
In a wide class of transmission line bipolar pulse generators, all transmission lines are of equal electrical length and are charged with equal voltage. This voltage is limited by fast rise time photoconductive switches and, consequently, by an optimally chosen transmission line with lowest characteristic impedance.
Each transmission line stores energy that is proportional to the inverse of the line's characteristic impedance. To compare different generator's circuits, the total energy stored in all transmission lines can be determined in relation to the energy stored in the transmission line with lowest characteristic impedance. Lower characteristic impedance implies less dielectric thickness between the line's conductors and, therefore, a higher electric field, which is a limitation for selected charging voltage defined by the chosen switch.
Other aspects of structure selection of pulse generators are: suitable switch positions and the potential existence of shunt inductive stub for efficient usage of transmission line transformers.
Still another aspect of structure selection of a pulse generator is the position of an inductive stub, which can be incorporated with a transformer having an optimum ratio of load impedance and impedance of a shunt inductive stub. Some physical structures of bipolar pulse generators need a short-circuited stub, for example, as a resonant cavity.
No known bipolar pulse generators, however, have all desirable factors: 1—Maximum stored (transferred to the load as a pulse) energy; 2—Efficient impedance transformation; and 3—Inductive stub incorporable with a transformer; and 4—Simplicity of design and compactness.
Accordingly, there remains a need for a bipolar pulse generator solution based on voltage charged transmission lines, which is capable of implementing high energy/power and required impedance transformation ratios. There remains a further need for a bipolar pulse generator that combines three properties: maximizes stored energy transferred to the load as a pulse, easy to implement and that is able to efficiently transform the load impedance of the generator to a higher level of impedance compared to the lowest characteristic impedance of generator's transmission line. There is a further need for a bipolar pulse generator which is capable of being implemented in a compact structure. There is a future need for a bipolar pulse generator in which inductive stub of transformer is a circuit element of a generator with high energy/power, and which does not deteriorate generating pulse shape. There is still a further need for a bipolar pulse generator in which impedance of an inductive stub shunting the resistive load impedance may be selected for an optimum value.